Wind turbines create power proportional to the swept area of their blades. The choice of rotor attributes for a wind turbine, such as its diameter, is a design trade-off between longer blades for more energy production in low winds and shorter blades for load limitation in high winds. Thus, wind turbines having longer blades will increase the swept area, which in turn produces more power. However, at high wind speeds, or in turbulent winds, a wind turbine having longer blades places greater demands on the components and creates more situations where the turbine must be shut down to avoid damaging components. Even in situations where the average wind speed is not high enough to cause damage, periodic wind gusts which change both the speed and direction of the wind, apply forces that may be strong enough to damage equipment.
Approaches with varying levels of success have been attempted in achieving higher power, fewer shut downs, and fewer instances of damage to components. For example, pitch control has been used to vary the pitch of the blade (i.e., the angle of the blade). On a pitch controlled wind turbine, an electronic controller on the turbine checks the power output of the turbine. When the power output exceeds a certain threshold, the blade pitch mechanism turns the rotor blades to reduce the loads on the rotor blades. The blades are later turned back when the wind drops again. However, pitch control can be fairly slow to respond to changes in the wind and is relatively ineffective to loads imparted by sudden wind gusts.
Stall control is another approach that has been used in an attempt to achieve higher power, and to reduce shut downs and damage to components. In passive-type stall controlled wind turbines, the rotor blades are mounted to the hub at a fixed angular orientation. The stall control is achieved passively by the shape of the blade being such that the blade goes into aerodynamic stall (destroying lift) when the wind speed exceeds a certain threshold. Active-type stall controlled wind turbines exist. In such systems, the rotor blades are adjusted in order to create stall along the blade. However, both types of stall control systems can be difficult to optimize and slow to respond, and may suffer from lower predictability of results than desired. These drawbacks are magnified in conditions with erratic winds and wind gusts.
Variable length rotor blade systems have also been used as an attempt to achieve higher power, and experience fewer shut downs and less damage to components. In such systems, the wind turbine rotor blades are telescopic so that their length can be adjusted based on the wind speed. This provides advantages in that the rotor blades can be extended to provide higher output in low wind conditions and retracted to lower loads in high wind conditions. U.S. Pat. No. 6,902,370 discloses a wind turbine system having telescoping wind turbine rotor blades. While variable length rotor blade systems have certain advantages, they may suffer drawbacks in erratic wind conditions or may be too slow to respond when experiencing a wind gust.
While some proposed turbine control systems use controls that vary blade pitch, speed or length to respond to and adjust power capture and loading of the rotor (see, e.g., U.S. Pat. No. 6,726,439 to Mikhail et al.), it is possible to control turbines with other inputs, such as electrical current; remote inputs from central controllers, signals from sensors not located on the turbine being controlled, or loads sensed on nearby turbines or structures. Additionally, control responses can be modified based on one or more of the following: Utility rates, storm warnings, manual over rides, wind farm physical production limits, wind farm contractual limits, noise limits, operator's long term goals, or utility curtailments. Currently, turbine controls do not integrate these types of inputs, and thus fail to optimize turbine production or to minimize turbine shutdowns and damage due to overloads.
As electricity continues to become a more valuable commodity, and as wind turbines present an environmentally-friendly solution to solve electricity shortage problems, a wind turbine design that overcomes the aforementioned drawbacks and provides increased power and decreased turbine shut downs and damage to components is thus desirable.